Average ΣPAH concentrations and composition at the Seep site. Average ΣPAH concentrations (top) and composition (bottom) at the Seep site at ~1400 m depth during 2012–2013. PAHs are colored as shown in the inset legend, along with interpretation of potential source. Seep site samples were grouped into five periods, according to seven hydrocarbon indices (see Methods), as containing clear signs of fresh crude oil (A1), weathered crude oil (A2), predominantly combustion products (B), no unusual contamination but an old isotope signal (C), or no unusual contamination (D). Average ΣPAH concentrations and composition at the Reference (Ref) site (April 2012–April 2013) and at the DWH site during June–September 2012 (DWH 2012) and August 2010–September 2011 (DWH 2010) are shown for comparison. Error bars show SD of cups within each period. DOI: https://doi.org/10.1525/elementa.264.f5